News Release Number: STScI-2003-10

March 26, 2003: In January 2002, a dull star in an obscure constellation suddenly
became 600,000 times more luminous than our Sun, temporarily making
it the brightest star in our Milky Way galaxy. The mysterious star,
called V838 Monocerotis, has long since faded back to obscurity. But
observations by NASA's Hubble Space Telescope of a phenomenon called
a "light echo" around the star have uncovered remarkable new features.
These details promise to provide astronomers with a CAT-scan-like probe
of the three-dimensional structure of shells of dust surrounding an
aging star.

Q & A: Understanding the Discovery

1.
What is a light echo?

It is light from a stellar explosion echoing off dust surrounding the
star. V838 Monocerotis produced enough energy in a brief flash to
illuminate surrounding dust, like a spelunker taking a flash picture of
the walls of an undiscovered cavern. The star presumably ejected the
illuminated dust shells in previous outbursts. Light from the latest
outburst travels to the dust and then is reflected to Earth. Because of
this indirect path, the light arrives at Earth months after light from
the star that traveled directly toward Earth.

2.
Why did the star produce this outburst?

Astronomers do not fully understand the star's outburst. It was
somewhat similar to that of a nova, a more common stellar outburst. A
typical nova is a normal star that dumps hydrogen onto a compact white-dwarf companion star. The hydrogen piles up until it spontaneously
explodes by nuclear fusion  like a titanic hydrogen bomb. This
exposes a searing stellar core, which has a temperature of hundreds of
thousands of degrees Fahrenheit.

By contrast, V838 Monocerotis did not expel its outer layers. Instead,
it grew enormously in size. Its surface temperature dropped to
temperatures that were not much hotter than a light bulb. This behavior
of ballooning to an enormous size, but not losing its outer layers, is
very unusual and completely unlike an ordinary nova explosion.

The outburst may represent a transitory stage in a star's evolution
that is rarely seen. The star has some similarities to highly unstable
aging stars called eruptive variables, which suddenly and unpredictably
increase in brightness.